The present invention relates to cleaning of contaminants from pieces of thermoplastic materials preparatory to reuse of such materials in molding processes.
Production of thermoplastic materials from raw materials by chemical processes is somewhat expensive, and it is desirable to keep waste of the materials at a minimum. Nevertheless, imperfections in molded products, particularly blow molded bottles for use in containing consumer goods, result in a considerable amount of plastic material which manufacturers would like to be able to reuse.
A problem which often interferes with reuse of such thermoplastic material is that labels are often applied to molded bottles before it has been determined that the bottles are unacceptable, particularly where labels are applied during the process of molding the bottles. Labels may be applied by adhesives which are water soluble at elevated temperatures, by pressure sensitive adhesives, or by thermal fusion of thermoplastic labels to bottles manufactured of the same sort of plastic. When such bottles or other plastic products are rejected, the plastic must be cut into small pieces to reduce its bulk for convenient handling. These pieces cannot simply be melted and reused, however, because attached pieces of label material would plug molding equipment or cause visible imperfections in products molded from such reused plastic material.
It has previously been attempted to clean such plastic materials by washing processes using quantities of water together with detergents or other chemicals, but such processes require rinsing, dewatering, and thorough drying of the plastic before it can be used in a molding process. Additionally, such previously available water-based washing processes did not clean pieces of plastic effectively enough, and too many remaining scraps of labels or other materials might foul molding equipment, causing production delays. Furthermore, previously known methods and apparatus for cleaning plastics require and thereby contaminate significant quantities of water, placing increased demands on available water supplies.
In general, previously available washing methods of preparing plastics materials for reuse have been too costly to be economically practical, either for small scale use in a factory where molding is performed, or for larger scale use in a central location to which several molders might send plastics material resulting from rejected production. Thus, it has previously remained less expensive to use new plastic material exclusively than to reuse material from rejected products.
Previously available machines have been used to densify clean low-density thermoplastic materials such as plastic foam, film, fiber, and the like by raising the temperature of such materials through friction, to the point where small pieces of material melt together. The agglomerated pieces are then quenched by a spray of water at ambient temperatures, forming lumps which can be handled easily. Materials to be densified using such machines first have to be adequately cleaned, nevertheless, in order for the agglomerated plastic to be useful in molding.
What is still needed, therefore, is an economical method and efficient apparatus for cleaning pieces of thermoplastic materials by removing contaminants such as labels and adhesives used to attach such labels to products such as bottles. Such a method and apparatus must be capable of reliably removing all but a completely insignificant portion of such contaminants resulting from normally occurring production imperfections, at a cost which makes such cleaning and use of the cleaned plastic material economically sensible.